Ultrasound transducers are widely used in many different fields, including ultrasound imaging. In conventional modern medical imaging applications, ultrasound transducers are typically constructed from piezoelectric materials. One commonly used piezoelectric material is lead zirconate titanate (PZT). However, the impedance of PZT is usually higher than 30 MRayls, while the impedance of human tissue is approximately 1.5 MRayls. To reduce this large impedance mismatch, one or more matching layers may be placed between the PZT transducer and the tissue being imaged. Since the matching layers are typically selected based on the one-quarter-wavelength principle, the bandwidth of PZT transducers having matching layers may be limited to 80% or less bandwidth.
Capacitive micromachined ultrasonic transducers (CMUTs) have been developed for various applications including medical ultrasound imaging. CMUTs can be used without matching layers and therefore can operate with extremely wide bandwidths (e.g., greater than or equal to 100%). Similar to a PZT transducer, a CMUT transducer may be activated by electrical pulses to generate acoustic signals that propagate in tissues; however, unlike a PZT transducer, a CMUT transducer may employ an additional bias voltage, such as when receiving echo signals from tissues. Traditionally, the bias voltage may be a DC voltage that remains constant during imaging operations.
Additionally, ultrasound transducer arrays may be classified into three or more different array types, which include one-dimensional (1D) arrays, one-point-five-dimensional (1.5D) arrays, and two-dimensional (2D) arrays made up of multiple CMUT elements. For example, a 1D array may include multiple CMUT elements arranged in only one dimension, e.g., the lateral dimension. The spacing between two adjacent elements may be typically either one wavelength for a linear array or one-half wavelength for a phased array. A 1.5D array may include multiple elements in the lateral dimension and at least two sub-elements in the elevation dimension. The spacing between the two adjacent sub-elements may be much larger than the wavelength. A 2D array may include multiple elements arranged in both the lateral dimension and the elevation dimension. As one example, the spacing between two adjacent elements may be one-half wavelength in both the lateral and the elevation dimensions. The number of elements and sub-elements of 1.5D arrays and 2D arrays may be significantly larger than the number of channels of the respective imaging systems.